Conventional X-ray systems produce radiographic projection images, which are then interpreted by an operator. These radiographs are often difficult to interpret because objects are superimposed. A trained operator must study and interpret each image to render an opinion on whether or not a target of interest, a threat, is present. With a large number of such radiographs to be interpreted, and with the implied requirement to keep the number of false alarms low, operator fatigue and distraction can compromise detection performance.
Advanced technologies, such as dual-energy projection imaging and Computed Tomography (CT), are being used for contraband detection, beyond conventional X-ray systems. In dual-energy imaging it is attempted to measure the effective atomic numbers of materials in containers such as luggage. However, the dual-energy method does not readily allow for the calculation of the actual atomic number of the concealed ‘threat’ itself, but rather yields only an average atomic number that represents the mix of the various items falling within the X-ray beam path, as the contents of an actual luggage is composed of different items and rarely conveniently separated. Thus dual-energy analysis is often confounded. Even if the atomic number of an item could be measured, the precision of this measurement would be compromised by X-ray photon noise to the extent that many innocuous items would show the “same” atomic number as many threat substances, and therefore the atomic number in principle cannot serve as a sufficiently specific classifier for threat versus no threat.
In X-ray CT cross-sectional images of slices of an object are reconstructed by processing multiple attenuation measurements taken at various angles around an object. CT images do not suffer much from the super-positioning problem present in standard radiographs. However, conventional CT systems take considerable time to perform multiple scans, to capture data, and to reconstruct the images. The throughput of CT systems is generally low. Coupled with the size and expense of CT systems this limitation has hindered CT use in applications such as baggage inspection where baggage throughput is an important concern. In addition, CT alarms on critical mass and density of a threat, but such properties are not unique to explosives. CT based systems suffer from high false alarm rate. Any such alarm is then to be cleared or confirmed by an operator, again interpreting images, or hand searching.
Apart from X-ray imaging systems, detection systems based on X-ray diffraction, or coherent scatter are also known. Their primary purpose is not to acquire images but to obtain information about the molecular structure of the substances an object is composed of. The so-called diffraction or coherent scatter signature is based on BRAGG reflection, that is the interference pattern of X-ray light, which develops when X-rays are reflected by the molecular structure or electron density distribution of a substance.
Various inspection region geometries have been developed and disclosed. Kratky, in Austrian Patent No. 2003753 publishes a refined arrangement of circular concentric apertures combined with an X-ray source and a point detector, to gain the small angle diffraction signature of an object placed between the apertures. More recently Harding in U.S. Pat. No. 5,265,144, uses a similar geometry but replaces the point shaped detector aperture with an annular detector configurations. Both patents are incorporated herein by reference.
The resulting diffraction spectra can be analyzed to determine the molecular structure of the diffracting object, or at least to recognize similarity with any one of a number of spectra, which have previously been obtained from dangerous substances.
One approach to detecting explosives in luggage was disclosed in British patent No. 2,299,251 in which a device uses Bragg reflection from crystal structures to identify crystalline and poly-crystalline substances. Substances can be identified because the energy spectrum distribution of the polychromatic radiation reflected at selected angles is characteristic of the crystal structure of the substance reflecting the radiation.
U.S. Pat. Nos. 4,754,469, 4,956,856, 5,008,911, 5,265,144, 5,600,700 and 6,054,712 describe methods and devices for examining substances, from biological tissues to explosives in luggage, by recording the spectra of coherent radiation scattered at various angles relative to an incident beam direction. U.S. Pat. No. 5,265,144 describes a device using concentric detecting rings for recording the radiation scattered at particular angles. Each of the prior art systems and methods, however, suffer from low processing rates because the scatter interaction cross sections are relatively small and the exposure times required to obtain useful diffraction spectra are long, in the range of seconds and minutes. For security inspections, equipment performance has to combine high detection sensitivity and high threat specificity with high throughput, at the order of hundreds of bags per hour.
U.S. Pat. No. 5,182,764 discloses an apparatus for detecting concealed objects, such as explosives, drugs, or other contraband, using CT scanning. To reduce the amount of CT scanning required, a pre-scanning approach is disclosed. Based upon the pre-scan data, selected locations for CT scanning are identified and CT scanning is undertaken at the selected locations. The inventors claim the pre-scan step reduces the scanning time required for each scanned item, therefore increasing throughput. However, the use of CT scanning is still inefficient, not threat specific, and does not allow for rapid scanning of objects.
U.S. Pat. No. 5,642,393 discloses a multi-view X-ray inspection probe that employs X-ray radiation transmitted through or scattered from an examined item to identify a suspicious region inside the item. An interface is used to receive X-ray data providing spatial information about the suspicious region and to provide this information to a selected material sensitive probe. The material sensitive probe, such as a coherent scatter probe, then acquires material specific information about the previously identified suspicious region and provides it to a computer. The disclosed system does not, however, address critical problems that arise in the course of applying a scatter probe to a selected suspicious region, including the accurate identification of a suspicious region, correction of detected data, and the nature of processing algorithms used.
Nuclear quadrupole resonance (NQR) is a contraband material detection device, which has applications in security screening. This technology has shown potential for the detection of a range of materials, in particular it is very effective for the detection of the types of explosives which can be the most challenging to detect using x-rays or CT machines. One potential weakness of the technique is that, with carefully designed electromagnetic shielding, the materials which it is being used to detect can be rendered undetectable. This potential problem is mitigated by the fact that such shielding consists of conductive (typically metal) volumes that must completely encapsulate the item to be detected. Because the items being searched for typically have a size large in comparison with most metal clutter (i.e. keys, coins, zippers, etc) the counter measure can be detected using a variety of metal detection techniques. However, the presence of a conductive loop around luggage means that the simplest forms of inductive metal detector would have limited performance.
Accordingly, there is need for an improved automatic threat detection and resolution system that captures data through an X-ray system and utilizes this data to identify threat items in a rapid, yet accurate, manner. There is also a need for determining the presence of potential shields of explosive materials. There is additionally a need to determine the shielding's size, volume, and position. Furthermore, there is a need for such detection technology to operate within enclosed metallic tunnels. Additionally, the system should provide for greater accuracy in utilizing pre-scan data to identify an inspection region and in processing scan data.